Unusual stars known as blue stragglers have been causing trouble for astronomers since they were first seen in 1953: they are hotter and brighter than they should be, and much younger too. Now, they are causing mischief again for astronomers that are trying to work out where they come from.

When astronomers observe stars from Earth (or from orbit) there are two properties they can measure: the colour of the star and its magnitude. Magnitude is a measure of how bright a star looks from here so is not intrinsic to the star. If we know the star's distance as well as its magnitude, though, we can work out its luminosity — how bright it really is. Astronomers can also take the colour of the star and work out its temperature. Anyone who has ever touched something that was "red hot" will be aware that colour and temperature are related, and so it is with stars. The hottest ones are blue and the coolest are red, but the spectrum of colours runs the full gamut between these — stars can be blue, white, yellow, red or anywhere in between.

Astronomers use the luminosity and temperature of a star to plot it on a Hertzsprung-Russell diagram (like the lovely hand drawn picture below). In any cluster of stars, most will sit along the middle stretch of the H-R diagram on the main sequence. The main sequence is where stars spend most of their life; our Sun is on the main sequence at the moment, has been there for just less than 5 billion years and has the same amount left to go. Once a star has exhausted all of its hydrogen, it will move off the main sequence — it does this at the 'turn off' — and evolve into a red giant.

All different colours of stars lie on the main sequence. The bluest, and hottest, ones still on the main sequence mark the point of the 'turn off' — past that, we're in straggler territory.

Blue stragglers are much younger than the other stars in their cluster and have masses larger than those at the main sequence 'turn off'. Their formation cannot be explained using our usual ideas of star formation.

As early as the 1960s, astronomers had begun thinking of explanations for these unusual stars. In 1964, astronomer William McCrea suggested that blue stragglers were created in close binary star systems where one star has already evolved into a giant and the other is still on the main sequence.

In binary systems, both stars orbit a common centre of mass. The distance between the stars varies and depends on where they are in their orbits. In close binary systems, the stars are near enough to each other that each star can influence the other's outer atmosphere.

In McCrea's paper, he suggested that a blue straggler is created when material at the surface of the giant gets pulled towards its smaller companion. The companion then grows as it steals more and more material from the giant star, eventually leaving the giant star with no outer layers. The giant collapses into a white dwarf, and the companion star is now larger, and brighter and hotter, than stars at the main sequence "turn off" — it is a blue straggler.

However, the strongest theory now seems to be that blue stragglers form when stars collide.

Both these hypotheses can be tested by looking for white dwarfs as binary partners to blue stragglers. Formations by collision would likely result in larger companions, but those arising from the transfer of mass from a giant star would almost certainly result in a white dwarf. If you try to look for white dwarfs next to blue stragglers, however, you'll quickly run into a problem: they are so small and faint that they cannot be observed directly. Astronomers had to find some way to indirectly search for white dwarfs.

Aaron Geller and Robert Mathieu, from the University of Wisconsin-Madison, did just that.

Geller and Mathieu looked at blue stragglers in the star cluster NGC 188. Twelve of the 16 blue stragglers in the cluster have extremely long orbital periods — taking over three years to orbit their binary companions. All but two take over 100 days, so these long period blue stragglers are the ones that Geller and Mathieu focussed on.

By looked at how the light from these blue stragglers changed over time, Geller and Mathieu were able to confirm that they did indeed have binary companions. The next, and trickier, step was to find out the mass of these companions. By assuming that the companion masses follow a particular distribution, they were able to compare the changes in light from the blue stragglers, caused by orbiting their companions, and come up with a mass distribution for the companions.

Their results show that the average blue straggler companion has a mass about half that of the Sun. Overall, the companion masses do not vary much from this average. This suggests only one thing: all of the blue stragglers in the NGC 188 cluster formed by mass transfer, not by collisions.

To test their observations against current theories, Geller and Mathieu set into motion a simulation of the cluster NGC 188, using current knowledge of stellar evolution (including that of stars in binary systems) and basing as many parameters as they could on real observations. After speeding through 7 billion (7,000,000,000) years of evolution, the simulation matched NGC 188 almost perfectly.

When they looked a little closer at the simulated blue stragglers, however, they found that about half of them had formed by mass transfer and half through collisions between stars. But this didn't match their observations of the real blue stragglers in NGC 188. In NGC 188, all of the blue stragglers formed through mass transfer.

Geller and Mathieu also couldn't quite rule out the hypothesis that some of the blue stragglers in NGC 188 formed by mergers between stars in triple systems. However, they think that this is quite unlikely and hope to rule it out once they have a chance to observe NGC 188 using the Hubble Space Telescope. They hope the Hubble observations will allow them to directly detect the light coming from the white dwarfs, too.

So why the difference between simulation and reality? Geller's and Mathieu's results suggest that our theories of stellar evolution, at least when the stars are close together, could do with some tweaking. As usual, the answer seems to be: it's a little more complicated than we originally thought.


Geller AM, & Mathieu RD (2011). A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions. Nature, 478 (7369), 356-9 PMID: 22012393